This work addresses the mechanisms by which the neural networks that control behavior are modulated by neurotransmitters and neurohormones. The proposed experiments are designed to determine how the modulatory substances in the nervous system a) modify the properties of individual neurons, and b) alter the ways in which neurons interact. The rhythmically active central pattern generating circuits of the crustacean stomatogastric nervous system provides an ideal experimental preparation for this work. Anatomical techniques are used to determine which neuromodulators are colocalized in identified neurons. Electrophysiological and biophysical recording methods are used to characterize membrane and modulator currents. A new method, the dynamic clamp, will be employed to modify the intrinsic properties of individual neurons and their connections to determine their influence on circuit dynamics. The movements resulting from circuit modulation will be studied. Together these approaches will explain how behavior is produced at the cellular level. Neural oscillators and oscillatory networks contribute to sensory and motor behavior, but relatively little is understood about how oscillatory neurons and networks function, and malfunction in movement disorders and epilepsy. The proposed work addresses a number of fundamental problems related to neural oscillators. Moreover, the mechanisms by which single neurons can be recombined into several different functional circuits are studied. These studies will aid our understanding of the ways in which the modulatory environment of the brain shapes brain function.